As silicon device sizes become increasingly smaller, it can be increasingly difficult to form gate insulators capable of maintaining the capacitance of a dynamic random access memory (DRAM) cell in the range of 30 femptofarads (fF). Such capacitance is desired to achieve immunity to radiation and soft errors, and to keep noise to acceptable levels.
A commonly utilized dielectric material is silicon dioxide (SiO2). However such can be difficult to utilize in the 30 fF regime because the dielectric becomes so thin that direct band-to-band tunneling current or Fowler-Nordheim tunneling current becomes problematic. Accordingly, there has been an effort to utilize high-k (i.e., high dielectric constant) films (such as Al2O3, Ta2O5, and TiO2), as materials to substitute for the very thin silicon dioxide as dielectric materials in semiconductor devices.
Of the listed high-k materials, aluminum oxide has received significant interest. Aluminum oxide has been used as a high-k inter-poly dielectric (IPD) for low voltage/high speed flash memories. Specifically, it has been shown that 10 nanometer thick Al2O3, with k of about 10, can reduce an erasing time by three orders of magnitude in comparison with 15 nanometer thick ONO (with ONO referring to a sandwich of silicon dioxide, silicon nitride, and silicon dioxide). The ONO has a k of about 10. Aluminum oxide can reduce the erasing voltage of a flash memory device by about 40% compared with silicon dioxide, and about 27% compared with ONO of the same thickness as the aluminum oxide.
Difficulties associated with Al2O3 films include that the optical properties of the films can vary significantly depending on the substrate that the film is grown on. For instance, studies have been done in which Al2O3 films have been grown by atomic layer deposition on Si, SiO2 and TiN, and the studies have found significant differences in the optical properties of the film depending on the underlying substrate.
There has recently been some investigation on the effects of dopant addition to aluminum oxide which indicates that particular dopants can improve the properties of aluminum oxide. Specifically, the studies indicate that either silicon or zirconium can be added to an aluminum material to form a doped Al2O3 film with low leakage current and high thermal stability (up to 800° C.), and with a dielectric constant greater than 8. It can be preferred to keep the dielectric constant high, and preferably to keep the dielectric constant above 10, as there is likely a limited opportunity for dielectric materials with a constant less than 10 to replace silicon dioxide. However, alternate gate dielectrics having dielectric constants greater than 10, and more preferably greater than 15, may ultimately be desired as replacements for the silicon dioxide materials currently being utilized.
Some work with aluminum oxide has focused on methods of deposition of thin films of aluminum oxide. For instance, there has been development of procedures for atomic layer deposition (ALD) of aluminum oxide to DRAM and FeRAM (ferroelectric random access memory). It is found that ALD films typically have lower leakage current and larger dielectric breakdown voltage than conventional thermal oxide materials. The leakage characteristics of MIS aluminum oxide capacitors are lower than that of MIS-Ta2O5 capacitors. Also, it is found that the ALD aluminum oxide can have low leakage and low interface surface state densities.
A method of forming thin aluminum oxide films is to evaporate sapphire (a form of high-purity aluminum oxide), and to subsequently condense the evaporated material on a substrate. Sapphire can be evaporated by, for example, electron gun evaporation or ion beam evaporation (also called ion beam deposition). It is noted that aluminum oxide films have also been deposited by thermal evaporation of single crystal sapphire. In some instances, aluminum oxide can be evaporated and deposited within a vacuum chamber under conditions in which no additional oxygen is admitted to the vacuum chamber during evaporation, and in which a substrate temperature is varied between 80° C. and 140° C. A typical deposition rate can be from about 10 Å per second to about 12 Å per second. The electrical breakdown voltage and resistivity can be improved at higher substrate temperatures. It is found that if the films are exposed to 98% relative humidity, the breakdown voltage can change to a significantly lower value, such as, for example, to a value of from about 106 volts/cm to about 104 volts/cm.
The aging properties of electron gun evaporated alumina films have been investigated by mass spectrometry and IR spectrometry. It has been found that if films are stored under normal atmospheric conditions, a characteristic absorption's peak will appear in the infrared reflection spectra which has been attributed to a build-up of water in the Al2O3 films. Such can cause the films to lose dielectric properties, and the breakdown voltage can decrease by several orders of magnitude.
Subsequent studies using electron-optical examination of the cross-section of vacuum-deposited aluminum oxide films shows that the films are amorphous on cold substrates. However, as the substrate temperature increases, an acicular crystalline structure appears for films having thicknesses on the order of 100 nanometers, with typical column diameters of from about 12 nanometers to about 17 nanometers. Such structure leads to high film porosity, with a considerable fraction of the pores extending through the entire thickness of the structure and being filled with water if a humidity is 70% and higher. Other reports have described a porosity of about 5% for films prepared at 60° C. The relative porosity of aluminum oxide is also evidenced by examination of the protective properties of aluminum oxide layers (100 nanometers) prepared at room temperature on silver mirrors. Specifically, the films are found to not exhibit protective properties against corrosive agents such as water and hydrogen sulfide.
Studies of electron-beam evaporated aluminum oxide films grown under oxygen ion bombardment indicate that an index of refraction, and accordingly the film density, first rises, and then decreases with increasing ion current density for substrate temperatures between 70° C. and 250° C. Ion bombardment during deposition is thought to cause the film to become more compact by inhibiting columnar growth.
Similar investigations on the effect of plasma activation on the phase transformation of aluminum oxide films have been done with films deposited by reactive high-rate electron beam evaporation. Plasma activation of the vapor took place via a hollow-cathode plasma. Aluminum oxide films deposited at 500° C. and 700° C. without plasma activation were characterized by columnar structure, and exhibited a relatively high porosity. However, films deposited with plasma activation at the same temperatures showed glassy fracture and a much denser microstructure.
It would be desirable to develop improved methods for forming aluminum oxide films suitable for utilization as dielectric materials in semiconductor structures, such as, for example, aluminum oxide films suitable as dielectric materials in transistor gate structures and flash memory device structures.